The invention relates to an internal combustion engine for a portable handheld work apparatus such as a motor-driven chain saw, cutoff machine, brushcutter or the like.
An internal combustion engine of this kind is disclosed in international patent publication WO 98/57053 and is provided especially for use in portable handheld work apparatus such as motor-driven chainsaws, cutoff machines, brushcutters, blowers, overhead branch cutters or like work apparatus. The usual configuration comprises essentially a cylinder and a crankcase arranged below the cylinder. The piston moves reciprocally in the cylinder and drives a crankshaft rotatably journalled in the crankcase. For operating the engine, an air/fuel mixture is supplied to the crankcase via a carburetor. With a downward stroke of the piston, the mixture flows into the combustion chamber via transfer channels formed in the cylinder wall. In order to minimize scavenging losses, the transfer channels are connected to a bypass channel approximately at the elevation of the transfer windows and this bypass channel conducts exclusively fuel-free air. The bypass channel opens into a channel segment of the transfer channel next to the transfer window via a membrane valve.
A spark plug is necessary for operating the engine and is mostly mounted in the roof of the combustion chamber and is driven by an ignition control device. An ignition control device of this kind is disclosed, for example, in U.S. Pat. No. 4,483,279 and comprises a stationary ignition transformer which is fixed on the cylinder of the engine and operates together with an ignition magnet which rotates with the crankshaft. The ignition magnet is advantageously integrated into the fan wheel for moving cooling air to the engine and is flange-connected to the end of the crankshaft. For this reason, the ignition transformer must be fixed in a predetermined position on the cylinder.
In two-stroke engines having charge stratification or scavenging advance, the bypass channel for the supply of the fuel-free air lies approximately at half elevation of the cylinder whereby the attachment possibilities for an ignition transformer of the ignition control device are limited.
It is an object of the invention to provide an internal combustion engine of the above-mentioned type which is so improved that a reliable and permanently fixed arrangement of the ignition module is ensured. It is a further object of the invention to provide this permanently fixed arrangement of the ignition module with only slight additional constructive complexity.
The internal combustion engine of the invention is for a portable handheld work apparatus including a motor-driven chain saw, cutoff machine, brushcutter or the like. The internal combustion engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; a mixture forming device for supplying a fuel containing mixture to the combustion chamber; a channel segment formed in the cylinder wall and communicating with the combustion chamber via a controlled window; a connecting piece fixedly attached to the cylinder; a bypass channel communicating with the channel segment via the connecting piece to supply essentially fuel free air to the combustion chamber; a spark plug arranged in the combustion chamber; an ignition device mounted in the region of the cylinder; and, the ignition device having a part thereof fixedly attached with at least one attachment point to the connecting piece.
With the arrangement of at least the one attachment point of the ignition module on the connecting piece of the bypass channel, the region of the transfer channel can also be utilized as an attachment region. The attachment point of the ignition module lies close to the crankcase. If a first attachment point is provided on the connecting cover and a second attachment point is provided on the cylinder, then a large spacing between the attachment points can be achieved constructively whereby the ignition module can be fixedly mounted on the cylinder. Attachment points which lie at a large spacing with respect to each other in the direction of the cylinder axis provide a high stability which is resistant also to the intense vibrations of the two-stroke engine over a long service life.
An imaginary connecting line between the attachment points can lie approximately parallel to the cylinder axis. An arrangement of the crankcase end attachment point offset in the peripheral direction to the cylinder-head end attachment point can be advantageous.
In a special embodiment of the invention, it can be sufficient to provide all attachment points of the ignition module on the connecting piece so that the ignition module can be mounted as an assembly unit with the connecting piece. This mounting can also take place in advance of fixing the cover on the cylinder.
In a further embodiment of the invention, each attachment point is defined by a screw dome which can be configured as one piece with the cylinder and/or as one piece with the connecting piece. The bypass channel lies between the attachment point viewed in the direction of the cylinder axis so that a large spacing can be formed between the attachment points in the direction of the cylinder axis.
Preferably, the attachment piece is configured as a cover which closes the channel segment open to the outside of the cylinder. Preferably, the connecting piece is fixed with threaded fasteners to the cylinder. The attachment point of the ignition module on the cover can lie close to the screw point of the cover.